1. Field of the Invention
The present invention relates to the processing of dispersions of alumina trihydrate in polyester resins and, more particularly, to a method for reducing the dispersion viscosity of alumina trihydrate in polyester resins.
2. Description of the Art
Hydrates, such as alumina trihydrate (Al.sub.2 O.sub.3.3H.sub.2 O), are used in large quantities as flame retardant and smoke suppressive fillers for plastics, particularly for thermosetting polyesters. Such materials inherently contain water of hydration which is slowly released therefrom by dehydration in a fire situation. Increased loading of the hydrate into the mixture necessarily increases the flame retardant and smoke suppressive qualities of the hydrate-filled plastic material.
It is well known that the presence of alumina trihydrate, as is the case with other fillers or pigments, has the adverse effect of increasing the dispersion viscosity of the hydrate in plastics, particularly polyester resins. Such increased viscosity is proportional to the amount of trihydrate in the dispersion. Although loading the mixture with more than 50% trihydrate may be desired, such a mixture exhibits an unworkable viscosity. Therefore, prior to the present invention, the trihydrate loading had been limited by viscosity.
The prior art, such as U.S. Pat. No. 3,733,283, taught that lactam polymerization is not affected by the presence of water. However, according to the present invention, the dispersion viscosity of alumina trihydrate in a polyester resin is significantly reduced by slight dehydration of the alumina trihydrate. The actual weight loss on ignition from 110.degree. to 1100.degree. C. (LOI) of alumina trihydrate is initially about 34.6 to 34.8%. In accordance with this invention, at least a fraction of a percent of the chemically bound water is removed, reducing the LOI by at least 0.2% and less than about 5.0% to a range of approximately from 29.8 to 34.6% to yield the desired viscosity decrease when the trihydrate is subsequently dispersed in the polyester resin.
It is known that hydrated fillers normally contain weakly bound surface water as well as tightly boudn water of hydration. It has also been taught, for example, in U.S. Pat. No. 3,733,283, that a hydrated filler should be preheated for a time sufficient to drive off substantially all of the weakly bound surface water when incorporating hydrated fillers into a polylactam composition.
According to the present invention, the dispersion viscosity of the alumina trihydrate in polyester resins is reduced without appreciably affecting the flame retardant and smoke suppressive qualities of the hydrate-filled polyester material.